This invention relates to gas-liquid contacting, and, in preferred embodiments, to removing SO.sub.2 from a gas mixture, e.g., from the flue gas of a power generator.
In the conventional Wellman-Lord SO.sub.2 scrubbing process a typical tower has from 3 to 5 absorber trays. The flue gas to be treated enters the base of the tower, and a sodium sulfite solution scrubbing liquid is fed to the top of the column. The liquid flows across the top tray, through a downcomer to the next tray, across that, and so forth until the solution exits from the bottom tray. The gas flows countercurrent to the liquid, and SO.sub.2 is absorbed via the chemical reaction: EQU SO.sub.2 + Na.sub.2 SO.sub.3 + H.sub.2 O.fwdarw.2 NaHSO.sub.3.
The bisulfite rich liquid leaving the tower is regenerated to sulfite solution by heating, driving off concentrated SO.sub.2 and precipitating sodium sulfite crystals.
In order to permit a low overall liquid feed rate while still maintaining an adequate liquid head on each tray in the face of liquid weeping and entrainment, conventional towers employ liquid recirculation at each stage. At each downcomer 90% of the liquid is recirculated to the tray inlet and only 10% passes to the tray below; thus, at each inlet 9 parts of recirculated liquid mix with one part of liquid from the tray above (or freshly regenerated liquid in the case of the top tray) for passage across the tray.
When oxygen in the flue gas contacts the scrubbing solution some of the sulfite is oxidized to Na.sub.2 SO.sub.4. The sulfate is non-regenerable and must be purged from the system, resulting in a loss of active sodium.
Other aspects of the Wellman-Lord process are described in a United States Environmental Protection Agency publication entitled: EPA-600/2-76-008, Jan. 1976, "SO.sub.2 Control Processes For Non-Ferrous Smelters", pp. 59--64, hereby incorporated by reference.